The present invention relates to ultra wideband (UWB) radar, communication, and/or geopositioning. More particularly, the present invention relates to a method and a system that improve UWB pulse transmission in order to enable an accurate measurement of pulse arrival time at a receiver.
UWB wireless technology has increasing application in communications, radar and precision geopositioning systems. A salient feature of UWB is that the transmitter produces short bursts of radio frequency (RF) energy at a relatively low duty cycle. That is, the ratio of the UWB pulse width (xcfx84) to pulse interarrival time (T) is much less than unity, with typical values for xcfx84/T being in the order of 10xe2x88x923 and often significantly less, as illustrated FIG. 1. In a specific UWB radar application, such as the Hummingbird collision avoidance sensor developed by the assignee hereof, a pulse having a width of 2.0 nanoseconds was generated using 500 MHz of instantaneous bandwidth with a pulse interarrival time of 100 microseconds (i e., 10,000 pulses per second). This produced a signal having a pulse duty cycle of 2xc3x9710xe2x88x925.
A direct consequence of a low duty cycle is that the average power transmitted is significantly lower than the peak power of an individual pulse. Indeed, the ratio of average to peak power is equal to the pulse duty cycle. A system described by Larrick, Jr. et al., in U.S. Pat. No. 6,026,125, incorporated herein, which is also assigned to the assignee hereof, takes advantage of this fact to provide a prime power reduction technique for ultra wideband systems in which xe2x80x9cactivation of the power amplifier may be time-gated in cadence with the UWB source thereby to reduce inter-pulse power consumption.xe2x80x9d Reduction of prime (e.g., battery) power is often critical to successful implementation of a wireless system, which typically operates remotely and untethered to a source of power. With power reduction (and, thus, extended battery life), reduced size, weight, and cost are often achieved as well. An additional aspect of the aforementioned ""125 patent is the generation of an ultra wideband RF signal by a direct impulse or step excitation of an intermediate or output bandpass filter. Thus, as predicted by the ""125 patent, any spurious step, impulse or wideband excitation can itself elicit the generation of spurious RF output energy.
A problem sometimes occurs with time-gated amplifiers as illustrated in FIG. 2. When a gate signal transition turns the amplifier on or off, spurious signal energy is sometimes produced at the amplifier output. Unfortunately, spurious outputs from a UWB transmitter may have a detrimental effect on the operation of a UWB wireless system as shown, for example, in FIGS. 3A and 3B, which illustrate a block diagram and typical waveforms associated with a gated UWB amplifier. Typically, the energy of a spurious signal waveformxe2x80x94itself the result of a fast switching transientxe2x80x94has a very wide bandwidth. If transmitted directly, it may interfere with wireless systems outside of the frequency band of authorization for the UWB source. Alternatively, the wideband spurious signal may also impulse excite the output bandpass filter of a UWB source, resulting in the generation of a spurious in-band RF signal. Such an in-band RF signal may create a false lock condition, which xe2x80x9cconfusesxe2x80x9d the receiver synchronization circuitry in a UWB communication application; or it may produce a false detection event at the receiver, which results in an inaccurate computation of the time-of-flight of a pulse in a UWB precision geopositioning application.
The present invention provides suppression of spurious signals, such as switching transients, that may occur in an ultra wideband gated RF transmitter. This may be analogous to a problem observed in audio frequency amplifiers in which switching transients in the amplifier supply voltage(s) induce spurious outputs that give rise to a loud xe2x80x9cpopxe2x80x9d or xe2x80x9cclickxe2x80x9d in the audio output. If serious enough, these transients sometimes result in physical damage to either the amplifier or connected speaker circuitry. Typical audio applications, however, do not require precise time gating, transient switching, or an ultra wideband spectrum required of UWB systems.
One solution to the audio amplifier problem has been proposed by U.S. Pat. No. 6,041,416. Typically, an audio amplifier is turned on and off only when the entire system power is turned on and off (e.g., power-down and power-up operations of a computer audio card during power management operations). By contrast, a gated amplifier used in an ultra wideband transmitter can be, and typically is, turned on and off for each arriving pulse to be amplified in order to minimize prime power requirements. In the UWB case, the gating signal often operates at an extremely high frequency (e.g., 100 kHz to several tens of MHz), and typically has an extremely low duty cycle, as described above.
RF push-pull amplifiers are known in the art, as evidenced by U.S. Pat. No. 4,455,536. One configuration of a typical push-pull RF amplifier is shown in FIG. 4. A basic principle of operation involves splitting an input signal into two components that are substantially equal in amplitude but opposite in phase, delivering these oppositely polarized signals to two substantially identical amplifiers, and subsequently recombining the two amplifier outputs into a single composite output signal. RF push-pull amplifiers typically use transformers to split and combine signals, and the transformers can be implemented in a variety of ways, including wire-wound transformers, transmission line transformers, baluns, and hybrid couplers.
In the prior art, several possible reasons exist for using a push-pull amplifier One reason is to increase the total power output by combining the outputs of multiple, smaller power output amplifiers. Another advantage of push-pull amplifiers relates to reducing distortion, as described in Uniplanar Broad-Band Push-Pull FET Amplifier by Hsu et al.xe2x80x94IEEE Transactions on Microwave Theory and Techniques, Vol 45, No. 12, December, 1997. A push-pull amplifier balances out even harmonics in the output and will leave the third harmonic as the principal source of distortion, thus possessing inherent spurious signal rejection of even order distortion.
In view of the foregoing, one objective of the present invention is to reduce overall transmitter power consumption by gating a balanced amplifier so that it is in a power-down state during inter-pulse intervals.
A further objective of the invention is to substantially eliminate spurious output signals during power-up and power-down of a UWB amplifier.
In accordance with the invention, there is provided a method of canceling or reducing spurious or similar output signals associated with a UWB transmitter where a UWB pulse is transmitted during an interval defined by switching events. The preferred method comprises splitting signals appearing at an amplifier input into signals of opposite phase, push-pull amplifying the signals of opposite phase, and combining the amplified signals whereby to reduce or cancel the spurious signals in the combined signal. The method may further include applying power to amplify the signals of opposite phase during push-pull amplifying and gating power applied during the amplifying step in accordance with time instances defining the amplifier switching events.
In accordance with another aspect of the invention, a switched push-pull amplifier free of spurious signal outputs is provided in a UWB pulse transmitter The preferred push-pull amplifier comprises a signal splitter that receives a low-level UWB signal at an input thereof and that divides the input signal into first and second signals having a substantially opposite phase relationship; respective amplifier segments of the push-pull amplifier that amplify the first and second signals to produce first and second amplified signals; a switch that responds to a gate signal to activate the push-pull amplifier; and a combiner that combines the first and second amplified signals to provide a composite output signal, whereby to provide cancellation or reduction of the spurious signal outputs resulting from powering up or powering down the amplifier. The splitter and/or the combiner may be implemented by a wire-wound transformer, a transmission line transformer, a balun, or a hybrid coupler.
The invention advantageously employs a push-pull amplifier or similar spurious signal canceling configuration that preferably operates to cancel or reduce spurious signal outputs before and/or after normal UWB pulse transmission. This permits more effective shut down of the UWB pulse generation or detection circuitry during null periods and start-up during time-gated UWB pulse transmission or detection windows. An important objective being not to increase power, or reduce harmonic distortion, but rather to eliminate the spurious output signals associated with gating an amplifier.
Other features, aspects, and advantages of the invention will become apparent upon review of the following disclosure taken in connection with the accompanying drawings. The invention, though, is pointed out with particularity by the appended claims.